Generation and protective efficacy of a cold-adapted attenuated genotype 2b porcine epidemic diarrhea virus

Won, Hokeun; Lee, Dong-Uk; Jang, Guehwan; Noh, Yun-Hee; Lee, Seung-Chul; Choi, Hwan-Won; Yoon, In-Joong; Yoo, Han Sang; Lee, Changhee

To assess the effectiveness of the cold-adapted live attenuated vaccine, a homologous primeboost sow immunization trial consisting of oral administration of a double-dose of the P29-CA virus (L/L vaccination) was implemented at intervals of 2 weeks before delivery. Newborn piglets born to vaccinated and unvaccinated sow groups were inoculated orally with the parental virulent PEDV strain. The difference in the burden of diarrheic disease as measured by mortality and morbidity was investigated between the challenged litters from vaccinated and unvaccinated sows. Piglets born to unvaccinated sows presented a much higher mean CSS, as determined by the intensity of diarrhea, than those of vaccinated sows. After 2 DPC, the challenged animals born to vaccinated (group 1) sows had mild-to-moderate diarrhea, whereas the piglets born to unvaccinated (group 2) sows experienced fatal watery diarrhea. Although nearly all the piglets in group 2 died by 7 DPC, group 1 also exhibited more than 30% neonatal mortalities during the experimental period. On the contrary, our previous trial has shown that a multiple-dose parenteral immunization with inactivated killed G2b vaccines (K/K vaccination) offered a more protective efficacy (93% survival rate) in piglets against G2b virus than that in the present study [39] . In contrast to piglets from unvaccinated sows that discharged high amounts of the virus in feces until death, PEDV fecal shedding in animals from vaccinated sows was greatly reduced at 7 DPC and more than 50% of the piglets that survived shed no virus in stools at the end of the trial. Collectively, the maternal L/L vaccination described in the present study is unlikely to be more efficacious than the K/K treatment in terms of the protective rate; however, it could help alleviate diarrheal severity, including the duration and quantity of fecal shedding of PEDV. Although our L/L vaccination of sows did not entirely avert morbidity in piglets upon a PEDV challenge, its advantageous effects on relieving PEDV shedding would lessen the possibility of the environmental contamination in the farrowing barns, thereby breaking the chain of secondary spread of the virus to other animals and herds during epidemics. Furthermore, PEDV transmission via the L/L vaccination is unlikely, as no PEDV genetic material was detected in feces from any of the pregnant sows vaccinated with P29-CA. Moreover, we were able to confirm the presence of satisfactory amounts of anti-PEDV neutralizing antibodies in the sera and colostrum of L/L vaccinated sows at farrowing and post-farrowing stages, as well as in sera obtained from their offspring, indicating the piglet protection by transfer of maternal immunity via mammary secretions from the immunized dams. These data further strengthen the concept that an optimal vaccination regimen is associated with retaining high levels of neutralizing antibodies in the lactogenic secretions of vaccinated sows, which in turn are correlated with colostrum and milk IgA antibody titers to offer protective immunity against PEDV [1, 2, 15, 40] .